Bug 1480454 - Part b: Improve assertion in _emit_manifest_at; r=bbouvier

This adds a message to the assertion, and removes a leftover pdb invocation
(which causes unnecessary delays if it is triggered in automation).

js/src/tests/lib/manifest.py

@@ -241,10 +241,7 @@ def _emit_manifest_at(location, relative, test_gen, depth):
             _emit_manifest_at(fullpath, relpath, test_list, depth + 1)
         else:
             numTestFiles += 1
-            if len(test_list) != 1:
-                import pdb
-                pdb.set_trace()
-            assert len(test_list) == 1
+            assert len(test_list) == 1, test_list
             line = _build_manifest_script_entry(k, test_list[0])
             manifest.append(line)